1. Field of the Invention
The invention relates to a control unit for an electrical-motor-driven power steering apparatus used for an automotive vehicle and so on.
2. Description of the Related Art
FIG. 16 is a block diagram schematically showing a conventional control unit according to an electrical-motor-driven power steering apparatus used for an automotive vehicle and so on.
With a steering shaft 42 connected to a steering wheel 41, there is provided a torsion bar 43 to which a torque sensor 44 is secured. In a force acts on the torsion bar 43 upon rotation of the steering shaft 42, the torsion bar 43 is twisted in correspondence with the added force, and its torsion of the torsion bar 43 is detected by the torque sensor 44.
To the steering shaft 42, there is fixedly attached a reducer 45 with which a gear 47 joined with a rotational axis of a motor is engaged. Further, a pinion shaft 48 is secured to the reducer 45. At a top portion of the pinion shaft 48, there is fixed a pinion 49 with which a rack 51 is engaged.
At both ends of the rack 51, there is secured each of tie-rods 52. Each of knuckles 53 is rotatably connected to each both ends of the tie-rods 52. A front wheel 54 is fixed to the knuckle 53. Further, the knuckle 53 is rotatably connected to a cross-member 55.
Accordingly, when the motor 46 is rotated, the number of its rotation is reduced by the reducer 45 and the reduced rotation is transferred to the pinion shaft 48. Thereafter, the rotation is transferred to the rack 51 through a rack-and-pinion mechanism 50. The knuckle 53 connected to the tie-rod 52 fixed to the rack 51 moves rightward or leftward in correspondence with a rotational direction of the motor 46. Besides, a vehicle speed sensor 56 is provided on the front wheel 54.
The rotational number and direction of the motor 46 is determined by positive or negative assist current supplied from a motor drive unit 57. The assist current that the motor drive unit 57 supplies to the motor 46 is calculated by an assist current determination means 58 for controlling the motor drive unit 57. The assist current determination means 58 calculates a steering torque Th of the steering wheel 41 based upon a detection signal VT from the torque sensor 44, and a vehicle speed V based upon a detection signal from the vehicle speed sensor 56.
The assist current determination means 58 calculates the assist current based upon the calculated steering torque Th and vehicle speed V. This calculation is led from an assist map previously stored in a memory of the assist current determination means 58.
In the conventional control unit for the electrical-motor-driven power steering apparatus as constructed above, in a case that the vehicle travels on a snowed road or on a crust (eisbahn), its road-surface reaction force is small, so that the assist torque by the motor 46 becomes to excess. This phenomenon is caused by that the aforementioned assist map is set based upon a dry road. Namely, the dry road is a high-xcexc road having a high coefficient of friction in which the road-surface reaction force is large, so that the assist force is set corresponding to the reaction force of the high-xcexc road by the assist map.
In the conventional control unit of the electrical-motor-driven power steering apparatus, supplemental controls such a handle-return control and a damper control exclusive of the aforementioned assist control are executed. In these control operations, the control command value is however calculated by the standard value obtained with dry road. Due to this control operation, in a case that the vehicle travels on a snowed road or on a crust (eisbahn), the road-surface reaction is small in the supplemental control, so that there is a problem such that a stability of the steering feeling is lacked.
In a control unit for an electrical-motor-driven power steering apparatus, there are provided an assist current command value calculated based upon a steering torque of a steering wheel, an assist current control value calculated based upon a motor current value of a motor and the assist current command value. A motor outputs an assist force based upon the assist current control value. Further, the control unit provides an estimation means for estimating a road surface xcexc based upon at least one of the motor current value and the steering torque.
To the aforementioned control unit, there are further inputted a vehicle speed obtained based upon rotation of a wheel, and a steering angle obtained based upon rotation of the steering wheel. Therefore, the estimation means estimates the road surface xcexc by comparing with a previously memorized standard value corresponding to the vehicle speed and the steering angle.
Further, the estimation means is executed when the vehicle speed and the steering angle exist within a predetermined vehicle speed range and within a predetermined steering angle range, respectively. A motor control means is also provided to control the motor based upon the estimated road surface xcexc. The road surface xcexc is estimated with the estimation means by executing a leveling processing taking account into the previously estimated road surface xcexc.
In the control unit, there is further provided an abnormal detection means for judging whether the estimated road surface xcexc is abnormal or not.
For the abnormal detection, the control unit includes a memory storing a formal road surface xcexc before the abnormal detection means detects the abnormal value, so that the estimation means can alternate a present road surface xcexc to the road surface xcexc when the abnormal detection means judges that the present road surface xcexc is the abnormal value.
Further, the estimation means respectively estimates the road surface xcexc corresponding to a right-steering operation and a left-steering operation.